A wireless local access network (WLAN) is a data transmission system to provide location independent network access between computing devices by using radio waves rather than a cable infrastructure. Often, WLANs are implemented as the final link between existing wired network and a group of client computers, giving these users wireless access to the full resources and services of the corporate network across a building or campus setting.
The rate at which wireless networks are being deployed is accelerating along with their size and ubiquity. Wireless networks using access points based on IEEE standard 802.11, commonly referred erroneously to as WiFi, and base stations based on IEEE standard 802.16 WiMax technology standards comprise a majority of current wireless deployments. There are also personal access networks deployed under the Bluetooth standard as well as other peer-to-peer network arrangements. Mobile connectivity under these standards is largely a matter of moving about within an area of coverage (i.e., a hot spot) in order to communicate with these network access devices.
The 802.11 specification as a standard for WLANs was ratified by the Institute of Electrical and Electronic Engineers (IEEE) in 1997. Like all of the IEEE 802 standards, 802.11 standards focus on the bottom two level of the International Organization for Standardization (ISO) model, the physical layer and the data link layer. The data link layer provides functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. In the current context, this data link layer is further subdivided into Media Access Control (MAC) sublayer that manages interaction of devices with a shared medium. Above the MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer that deals with addressing and multiplexing on multi-access media.
While such wireless capabilities offer a degree of mobility over wired infrastructure, coverage areas are still quite limited and disruptions in communication are frequent. Often, wireless communication is made on frequencies that are limited to essentially line of sight, subject to shadowing in coverage area due to topology and obstructions. Typically, selecting an access network is solely based upon received signal strength, which can roll off unexpectedly due to such unknown shadowing effects when moving to a new location and due to an unappreciated direction of travel away from the network device.
The 802.11 standards were implemented to provide reliable and secure wireless connectivity at high data rates. 802.11b and 802.11g standards use the 2.4 GHz band, operating in the United States under Part 15 of the FCC Rules and Regulations in the unlicensed Industrial, Scientific and Medical (ISM) bands. With the abundance of WLAN devices (e.g., access points, personal digital assistants (PDSs), laptop computers) in geographic proximity, interference is an increasing problem.
In addition, because of this choice of frequency band, 802.11b and 802.11g equipment could occasionally suffer interference from microwave ovens, sulfur lamps, wireless microphones, television broadcasts, or cordless telephones. Wireless personal area networks (PANs), such as Bluetooth devices, while operating in the same 2.4 GHz band, do not interfere with 802.11b and 802.11g in theory because they use a frequency hopping spread spectrum signaling method (FHSS) while 802.11b/g uses a direct sequence spread spectrum signaling method (DSSS). However, it should be appreciated that FHSS means that such devices should only occasionally collide on the same frequencies rather than not interfering at all. Physical and MAC layer adaptation is critical for performance wireless networks to mitigate the effect of interference. Previously, it has been difficult for the radio to detect what the exactly source of channel degradation and therefore may not take the right adaptation actions.
Even if a network access device is selected somehow that is most appropriate for a current location and route of travel, and even if correct adaptations are made to increase the effective coverage area, travelling at any rate at all tends to shrink the effective duration of the coverage to a vanishingly small amount of time. Many of these network access devices operate in an unlicensed frequency band and are thus regulated in the amount of power that can be broadcast. Consider the length of time that a person is within a hotspot, walking down the sidewalk. Further consider even how the larger coverage area of some municipal WiMax footprint shrink when transiting by car or rail through the coverage area.